1. Field of the Invention
The present invention relates to two-way radio communication systems of the type which use diversity, and to amplifier units placed in an antenna mast in such systems. The present invention is suitable for use especially in the base stations of cellular networks.
2. Background Information
In two-way radio communication systems, the transmitting and receiving branches of the system can have a shared antenna, but processing the signals, which propagate in opposite directions, naturally requires separate transmission paths for both. On the receiving side, the first amplifier starting from the antenna should be as close to the antenna as possible, because the receive signal is a low-level one and a long intermediate cable would attenuate it even more. Then, the signal-to-interference ratio in the input of the amplifier would be poorer compared to the amplifier being located close to the antenna. For this reason, the transmission path branches as viewed from the antenna in its vicinity to a transmitting and a receiving branch, and the latter includes a low-noise amplifier. When the antenna is close to the receiver and the transmitter, the transmission paths naturally continue separately to them. On the contrary, when the antenna is at the top of a mast relatively far away from the transmitter and the receiver, separate intermediate cables for transmitting and receiving cause a considerable additional cost for the apparatus. For this reason, transmission paths are usually joined again after the amplifier in the antenna mast so that only one intermediate cable comes down from the mast.
The term diversity refers to a means known for a long time for improving the reliability of radio communication. Types of diversity are, inter alia, frequency, polarization and spatial diversity. The term frequency diversity means that a signal containing the same information is transmitted using two carriers with different frequencies, and in receiving the signal of better quality is chosen. The term polarization diversity means that a signal containing the same information is transmitted using two carriers with the same frequency but orthogonally polarized, and in receiving the signal of better quality is chosen. Spatial diversity can be used in both transmitting and receiving. In transmission it means that a signal containing the same information is transmitted using at least two carriers with the same frequency, which are fed to antennas being located in different places, and in receiving the signals are summed. In receiving spatial diversity means that a radio signal is received by at least two antennas being located in different places, and the signals output by them are summed or the one of best quality is chosen.
FIG. 1 shows as a block diagram a typical known transmission path arrangement in a base station mast. The arrangement comprises two radiators 111, 112, front stages 120, 130 and intermediate cables 141, 142 leading down from the mast for implementing the diversity operation. The first front stage 120 comprises in parallel a receiving branch and a transmitting branch. The receiving branch comprises in the propagation direction of the signal a first receive filter 121, a low-noise amplifier 122 and a second receive filter 123. The receive filters are of band-pass type and they strongly attenuate frequency components outside the receive band. The transmitting branch only comprises a transmit filter 125 of band-pass type which strongly attenuates frequency components outside the transmit band. The input of the first receive filter 121 and the output of the transmit filter 125 are coupled by a shared line to the first radiator 111 of the antenna structure ANT. In this description the term “radiator” means an antenna element which may be for both transmitting and receiving. The output of the second receive filter 123 and the input of the transmit filter 125 are coupled to the shared first intermediate cable 141, which is connected at its lower end to a first base station segment BT1. The second front stage 130 is identical with the first front stage 120. Thus it comprises a receiving branch with a low-noise amplifier 132 between the receive filters 131 and 133 and a transmit filter 135 parallel to the receiving branch. The second front stage is coupled by a shared line to the second radiator 112 of the antenna structure ANT and from its end on the side of the base station to the shared second intermediate cable 142. This is connected at its lower end to a second base station segment BT2. The first 120 and the second 130 front stage together constitute an amplifier unit 100.
In FIG. 1 are further marked a transmit signal TX propagating via the transmit filter 125 to the first radiator 111 and a receive signal RX propagating from the first radiator to the amplifier 122.
FIG. 2 shows a disadvantage of the structure according to FIG. 1. The drawing presents one front stage according to FIG. 1. Similarly, it will be readily appreciated that the front stage has a first receive filter 221, a low-noise amplifier 222, and a second receive filter 223. Coupling the output of the second receive filter 223 to the input of the transmit filter 225 and coupling the output of the transmit filter to the input of the first receive filter in accordance with the above description produces a closed loop. This means that a certain part of the signal fed by the low-noise amplifier, 222 (LNA) circulates back to the input of the amplifier. In FIG. 2, the route of such a feedback signal, i.e. leak signal LK, has been drawn with a grey line. If the level of the leak signal becomes high enough, the loop starts to oscillate, in which case the front stage would be useless. However, also less fatal deterioration in operation can occur. For example, if the level of the leak signal is, e.g. 50 dB, lower than the level of the output signal of the LNA and the gain of the LNA is 30 dB, an “extra” gain ripple of almost one decibel occurs in the transmission curve of the receiving branch in its pass band, which ripple is far too high for desired operation. In order for such a gain ripple not to occur, the attenuation of the feedback path deducted by the amplification of the LNA should be 30-35 dB depending on the gain of the LNA. The required attenuation takes place in the transmit filter 225. This filter is designed so that it attenuates especially signals in the receive band. If the gain of the LNA is e.g. 32 dB, consistent with the above, a 67 dB attenuation is required of the transmit filter in the receive band. Such a high attenuation relatively close to the transmit band requires, for its part, a multi-resonator structure. Pass attenuation of the transmit filter i.e. its attenuation in the transmit band is naturally higher the more resonators there are in the filter in series. The pass attenuation is a critical parameter because all attenuation between the power amplifier of the transmitter and the antenna causes a decrease in the efficiency and heating of the power amplifier. In order for the pass attenuation of a multi-resonator transmit filter to remain, e.g., less than 0.5 dB, relatively large-sized resonators are required, i.e. a massive and thus expensive transmit filter is required. Ultimately, this disadvantage is caused by the loop in the front stage.